Optical information recording medium, method for manufacturing the same, and initialization device

ABSTRACT

An optical information recording medium includes a plurality of recording layers, a reflectance of at least one of the plurality of the recording layers in a non-initialized state with respect to a light beam for initialization being smaller than a reflectance of the same in an initialized state with respect to the light beam for initialization. The recording layers are initialized by irradiating, among the plurality of recording layers, the recording layer positioned farther from a light beam irradiation side with the light beam prior to irradiating the recording layer positioned closer to the light beam irradiation side, so that the initialized recording layers have no initialization irregularities. In initialization, at a position at a same distance in a radial direction on the optical information recording medium, the recording layer positioned farther from a light beam irradiation side is irradiated with one light beam before the recording layer positioned closer to the light beam irradiation side is irradiated with another light beam so that the plurality of recording layers are initialized at the same time by a light beam projecting operation in which the light beams are focused at different positions. This ensures the initialization without irregularities within a short time.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an optical information recordingmedium having a plurality of recording layers to/from which informationis recorded/reproduced by irradiation with laser light or the like, andan initializing method and an initialization device for initializing theoptical information recording medium.

[0003] 2. Related Background Art

[0004] Optical information recording media have drawn attention aslarge-capacity and high-density memories, and so-called erasable typesof the same, which are rewritable, are being developed now. The erasableoptical information recording media include a medium that includes as arecording layer a thin film that exhibits a phase transition between theamorphous state and the crystalline state, to which information isrecorded and erased by thermal energy generated by the irradiation withlaser light.

[0005] As the phase-change materials for the foregoing recording layers,films made of alloys composed of Ge, Sb, Te, In, etc. as principalcomponents are known, for instance, a GeSbTe alloy. Information isrecorded by forming recording marks, which are obtained by causing therecording layer partially to make a transition into amorphous phase.Generally, information is erased by crystallizing the recording marks inmany cases. The transition to the amorphous state is achieved by heatingthe recording layer to above a melting point of the same and thereaftercooling the same. On the other hand, the crystallization is achieved byheating the recording layer to a temperature above a crystallizationtemperature and below the melting point. Moreover, the recording layeris formed generally by sputtering, but a thin film of the aforementionedphase-change material formed by sputtering exhibits an amorphous statein most cases. Therefore, it is necessary to make the recording layerassume a crystalline state prior to recording information. This processis called “initialization”.

[0006] A conventional initialization device for performing the foregoinginitialization has one optical head equipped with one light source andone objective lens, and initializes a desired area of the recordinglayer by irradiating the recording layer of the recording medium with alight beam emitted from the optical head while moving the optical headin a predetermined direction.

[0007] On the other hand, recently, the volume of information handledincreases as the processing ability of information devices improves.Therefore, an increased-capacity recording medium capable ofrecording/reproducing at an increased speed has been demanded. Toachieve such an increased capacity and an increased speed, a multilayerrecording medium has been proposed, which is configured so as to includea plurality of recording layers, so that information can berecorded/reproduced to/from each of the recording layers from one sideof the medium (e.g. JP 9(1997)-91700A).

[0008] However, the initializing method according to JP9(1997)-91700Ainitializes the recording layer closer to the beam irradiation sidefirst, which results in a drawback in that the recording layers tend tohave irregularities. More specifically, portions initialized have lesstransmittance of light because of crystallization, which hinders therecording layer farther from the foregoing side from being initializeduniformly.

[0009] Furthermore, in the case where the foregoing multilayer recordingmedium is initialized by a conventional initialization device, only onerecording layer is initialized at once. Therefore, it is necessary torepeat the initializing operation a number of times equal to the numberof recording layers, which results in a drawback in that a time requiredfor the initialization increases significantly. Besides, whereas ashort-wavelength light beam is employed for the recording/reproductionof a recording medium to increase the recording density of the medium, along-wavelength light beam is employed for the initialization of therecording medium to increase a light intensity of the beam. Therefore, alight beam with a wavelength different from the optical designwavelength for the recording medium is used for the initialization,which destabilizes the focus servo that is for adjusting the focusposition precisely to the recording layer. This incurs initializationirregularities in the recording layer, thereby deteriorating theperformance of the recording medium. For a multilayer recording mediumin particular, it is necessary to bring the focal position precisely toa specific recording layer among the plurality of recording layers,thereby making the foregoing problem more severe.

SUMMARY OF THE INVENTION

[0010] The present invention is to solve the above-describedconventional problem, and it is an object of the present invention toprovide a method for manufacturing an optical information recordingmedium and an initialization device that are superior in thestabilization of the focus servo and allow production of a multilayerrecording medium having a plurality of recording layers within a shorttime, and to provide a high-performance multilayer recording mediumwithout initialization irregularities in recording layers.

[0011] To achieve the foregoing objects, an optical informationrecording medium of the present invention includes a plurality ofrecording layers, in which a reflectance of at least one of theplurality of the recording layers in a non-initialized state withrespect to a light beam for initialization is smaller than a reflectanceof the same in an initialized state with respect to the light beam forinitialization. The initialized recording layers have no initializationirregularities, by initializing the recording layers by irradiating,among the plurality of recording layers, the recording layer positionedfarther from a light beam irradiation side with the light beam prior toirradiating the recording layer positioned closer to the light beamirradiation side.

[0012] Next, a manufacturing method of the present invention, ininitializing the optical information recording medium having a pluralityof recording layers, includes projecting different light beams to theplurality of recording layers, respectively. With respect to theposition at the same distance in a radial direction on the opticalinformation recording medium, the recording layer positioned fartherfrom a light beam irradiation side is irradiated with one light beambefore the recording layer positioned closer to the light beamirradiation side is irradiated with another light beam so that theplurality of recording layers are initialized at the same time by alight beam projecting operation in which the light beams are focused atdifferent positions.

[0013] Next, another manufacturing method of the present invention is aninitializing method for initializing an optical information recordingmedium having a plurality of recording layers, in which, in initializingat least one of the recording layers, a focus position control lightbeam different from a crystallization beam for crystallizing a recordinglayer is employed for controlling a focus position of thecrystallization beam.

[0014] Next, an initialization device of the present invention is aninitialization device for initializing an optical information recordingmedium having a plurality of recording layers, and the device includes aplurality of optical heads provided to face a same surface of therecording medium for irradiating different recording layers with lightbeams, respectively, and a transfer system for moving the optical headsin a predetermined initialization traveling direction. In theinitialization device, among the plurality of optical heads, the opticalhead for irradiating the recording layer positioned farther from a lightbeam irradiation side among the plurality of recording layers ispositioned ahead relative to the initialization traveling direction.

[0015] Next, another initialization device of the present inventionincludes at least one optical head and a transfer system for moving theoptical head in a predetermined initialization traveling direction. Inthe initialization device, at least one of the optical heads includes anobjective lens that is a single lens, a light source for emitting acrystallization beam for crystallizing a recording layer, and a lightsource for emitting a focus position control beam with an intensity withwhich the recording layer is not crystallized.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]FIG. 1 is a partially cross-sectional view illustrating aninitialization device for use with an optical information recordingmedium according to a first embodiment of the present invention.

[0017]FIG. 2 is a view illustrating a configuration of a principal partof the initialization device for use with an optical informationrecording medium according to the first embodiment of the presentinvention.

[0018]FIG. 3 is a cross-sectional view illustrating a principal viewillustrating an optical information recording medium according to thefirst embodiment of the present invention.

[0019]FIG. 4 is a partially cross-sectional view illustrating aconfiguration of an initialization device for use with an opticalinformation recording medium according to the first embodiment of thepresent invention.

[0020]FIG. 5 is a partially cross-sectional view illustrating aninitialization device for use with an optical information recordingmedium according to a second embodiment of the present invention.

[0021]FIG. 6 is a partially cross-sectional view illustrating aconfiguration of a principal element of an initialization device of anoptical information recording medium according to the second embodimentof the present invention.

[0022]FIGS. 7A and 7B are waveform diagrams illustrating examples offocus error signals obtained from the initialization device for use withan optical information recording medium according to the secondembodiment of the present invention.

[0023]FIG. 8 is a partially cross-sectional view illustrating aninitialization device for use with an optical information recordingmedium according to a third embodiment of the present invention.

[0024]FIG. 9 is a partially cross-sectional view illustrating aninitialization device for use with an optical information recordingmedium according to a fourth embodiment of the present invention.

[0025]FIGS. 10A, 10B, and 10C are waveform diagrams illustratingexamples of focus error signals obtained from the initialization devicefor use with an optical information recording medium according to thefourth embodiment of the present invention.

[0026]FIG. 11 is a partially cross-sectional view illustrating aninitialization device for use with an optical information recordingmedium according to a fifth embodiment of the present invention.

[0027]FIG. 12 is a cross-sectional view illustrating an example of anoptical information recording medium of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0028] In the optical information recording medium of the presentinvention, it can be determined, even from a recording medium alreadyinitialized, that a reflectance of at least one of the plurality ofrecording layers in a non-initialized state with respect to theinitialization beam is smaller than a reflectance of the same in aninitialized state. This is because non-initialized portions remain in acentral part and/or peripheral part in an ordinary recording medium evenafter it is initialized.

[0029] It can be determined that the recording layers do not haveinitialization irregularities by measuring jitter (JIT) values of areproduction signal after information is overwritten several times, orby checking whether recorded information can be reproduced normally.

[0030] Furthermore, the reflectance of the at least one of the pluralityof recording layers in the non-initialized state with respect to thelight beam for initialization may be not more than 1/2 of thereflectance in the initialized state. In other words, the reflectanceafter the initialization is not less than two times the reflectance inthe non-initialized state.

[0031] Furthermore, the recording layer positioned farther from thelight beam irradiation side and the recording layer positioned closerthereto are initialized at the same time by irradiation with one lightbeam, by focusing the light beam at different positions.

[0032] Furthermore, in the manufacturing method of the presentinvention, by initializing the plurality of recording layers at oncefrom the recording layer farther from the light beam irradiation side bya light beam projecting operation in which different positions arefocused, the initialization without initialization irregularities can beachieved within a shorter initialization time. More specifically, in theforegoing initializing method, it is possible to initialize a multilayerrecording medium within a short time and to initialize a recording layerfarther from the light beam irradiation side without the influence ofinitialization of a recording layer closer to the light beam irradiationside.

[0033] The light beams may be emitted from a plurality of optical heads,respectively, that are provided so as to face the same surface of therecording medium, and are moved in a predetermined initializationtraveling direction, and among the plurality of recording layers, therecording layer positioned farther from the light beam irradiation sidemay be irradiated with the light beam emitted from the optical headpositioned ahead relative to the initialization traveling direction, inthe initialization of the plurality of recording layers at the sametime.

[0034] Furthermore, the light beams emitted from at least two opticalheads among the plurality of optical heads may form spots in differentshapes from each other.

[0035] Furthermore, in initializing at least one of the recordinglayers, a focus position control light beam different from acrystallization beam for crystallizing a recording layer is employed forcontrolling a focus position of the crystallization beam. This allowsthe initialization to achieve the stabilization of focus servo, and itis possible to detect a position of a recording film with accuracy.

[0036] In initializing at least one of the recording layers, at aninitial stage of the initialization, a focus position of an optical headis moved up and down, with a light beam having an intensity sufficientfor initialization, so that the recording layer is crystallizedpartially. The partial crystallization causes the crystallized portionsto have a higher reflectance, whereby a position of the recording filmcan be detected accurately. Consequently, the initialization method isallowed to achieve stabilization of focus servo even with respect to arecording medium whose recording layer has a small reflectance in anamorphous state.

[0037] In initializing at least one of the recording layers, at aninitial stage of the initialization, a focus position is focused at arecording layer from which a greater reflected light amount may beobtained than a reflected light amount obtained from the targetrecording layer, and thereafter a focus position of an optical head ismoved a predetermined distance in a thickness direction of the recordingmedium so that the target recording layer is initialized. This methodalso allows the position of the recording layer to be detectedaccurately.

[0038] A focus position may be focused at a recording layer from which agreater reflected light amount is obtained than a reflected light amountobtained from the target recording layer, with a light beam having anintensity with which the target recording layer is not crystallized.Thereafter a focus position of an optical head may be moved up and downwith respect to a position at a predetermined distance therefrom in athickness direction of the recording medium so that the target recordinglayer is crystallized partially. This method allows the position of therecording film to be detected accurately.

[0039] A focus position may be focused at a recording layer from which agreater reflected light amount is obtained than from the targetrecording layer, with a light beam having an intensity with which thetarget recording layer is not crystallized. Thereafter a focus positionof an optical head is moved a predetermined distance in a thicknessdirection of the recording medium, and the intensity of the light beamis increased so as to be sufficient for initializing the targetrecording layer, so that the target recording layer is initialized. Thismethod also allows the position of the recording layer to be detectedaccurately.

[0040] A positional relationship of the optical heads may be set so asto satisfy a relationship expressed as:

z>(x/2)+(y/2)+(d·tan θ)

[0041] where x represents a spot width of a light beam for initializingthe recording layer positioned closer to the light beam irradiationside, y and θ represent a spot width and an incident angle,respectively, of a light beam for initializing the recording layerpositioned farther from the light beam irradiation side, z represents adistance between the light beams, and d represents a distance betweenthe recording layers. This causes the recording layers to be initializedindividually even if the plurality of recording layers are initializedsimultaneously, and are not subjected to the influence of anotherinitialization operation. Therefore, it is possible to preventinitialization irregularities from occurring.

[0042] Next, an initialization device of the present inventioninitializes a multilayer recording medium within a short time, and makesit possible to initialize a recording layer positioned farther from thelight beam irradiation side without the influence of initialization ofthe recording layer positioned closer to the light beam irradiationside. The device preferably includes a spindle motor for rotating therecording medium, a plurality of optical heads for irradiating differentrecording layers in the recording medium with light beams emittedtherefrom, respectively, a transfer mount on which the optical heads aremounted, and a transfer system for moving the transfer mount to apredetermined position. The plurality of optical heads are arranged sothat light beams emitted from the optical heads are projected to therecording medium at different positions in a radial direction of therecording medium.

[0043] Furthermore, the plurality of optical heads preferably aremounted on one transfer mount. This allows the device to be simplifiedin configuration and reduced in size.

[0044] Furthermore, optimal substrate thicknesses for the optical headsmay be different from each other, the optimal substrate thicknessesbeing obtained by optimizing substrate thicknesses through which thelight beams emitted from the optical heads pass while being converged toform spots on the recording layers, respectively, so as to make theformed spots have minimum diameters, respectively. This allows theoptical heads to focus light beams on target recording layers,respectively, whereby stable initialization without irregularities canbe achieved.

[0045] Furthermore, at least one of the plurality of optical heads mayinclude a driving system for moving an objective lens equipped in theoptical head, a counter for counting up-and-down movements of theobjective lens, and a controller for controlling an intensity of thelight beam emitted from the optical head and for controlling the drivingsystem. With this configuration, by moving the focus position of theoptical head up and down to partially crystallize the recording layerwith a light beam having an intensity sufficient for initialization, thedevice is capable of carrying out initialization with stable focusservo, even with respect to a recording medium having an extremely smallreflectance of the recording layer in amorphous state.

[0046] Furthermore, the optical heads may be provided facing the samesurface of the recording medium and project light beams to differentrecording layers, respectively, and among the plurality of opticalheads, the optical head for irradiating the recording layer positionedfarther from a light beam irradiation side among the plurality ofrecording layers may be positioned ahead relative to the initializationtraveling direction.

[0047] Furthermore, the foregoing optical head may include an objectivelens and a plurality of light sources, and the initialization devicefurther include an optical path correction system positioned on anoptical path of one light beam emitted from at least one of the opticalsources. This allows a light beam emitted from one light source to befocused on one specific recording layer by moving the objective lens,and allows a light beam emitted from another light source to be focusedon another recording layer by means of the foregoing optical pathcorrection system. In other words, even in the case where the distancebetween the recording layers in the recording medium is different fromthe designed value or in the case where the distance varies, the focusesof light beams emitted from a plurality of light sources can be focusedaccurately on target recording layers, respectively, at the same time,whereby stable initialization without irregularities can be achieved. Inthe foregoing configuration, the objective lens preferably is a singlelens.

[0048] Furthermore, the foregoing optical path correction systempreferably is a liquid crystal element or a lens.

[0049] Furthermore, the wavelengths of light beams of the plurality ofthe light sources may be different from each other.

[0050] As described above, the method for manufacturing an opticalinformation recording medium and the initialization device of thepresent invention allow a multilayer recording medium having a pluralityof recording layers to be initialized stably without irregularities,with stable focus servo within a short time. Furthermore, the opticalinformation recording medium of the present invention is ahigh-performance multilayer recording medium having recording layerswithout initialization irregularities.

EMBODIMENTS

[0051] The following will describe optical information recording media,methods for manufacturing the same, and initialization devices for usewith the same according to the present invention, while referring to thedrawings.

First Embodiment

[0052]FIG. 1 is a view illustrating a configuration of an initializationdevice for use with an optical information recording medium according tothe first embodiment of the present invention, and illustrates a statein which an information recording medium 1 having two recording layersis placed. The recoding medium 1 is formed in the following manner. Onan approximately 1.1 mm thick substrate 6 made of polycarbonate, a firstrecording layer 5, an approximately 0.04 mm thick transparent separationlayer 4, and an approximately 100 nm thick second recording layer 3composed of a semi-transparent layer are formed successively in thestated order, and further a protective film 2 is formed thereon. In therecording layers 3 and 5, guiding grooves (not shown) with a depth ofapproximately 20 nm and a width of approximately 0.2 μm are arranged ata track pitch of approximately 0.32 μm, for tracking a laser lightduring recording/reproduction.

[0053]FIG. 12 illustrates a more detailed structure of the recordingmedium 1. In FIG. 12, the recording layer 3 is a multilayer thin filmthat includes a protective layer 33 made of a dielectric material(ZnS—SiO₂, 50 nm thick), a phase-change layer 34 composed of a GeSbTethin film (7 nm thick), and a protective layer 35 made of a dielectricmaterial (ZnS—SiO₂, 40 nm thick). A transition of the phase-change layer34 from the amorphous state to the crystalline state upon initializationor the like increases a reflectance and decreases a transmittance of thesame. The recording layer 5 is a multilayer thin film that includes aprotective layer 36 made of a dielectric material (ZnS—SiO₂, 60 nmthick), a phase-change layer 37 composed of a GeSbTe thin film (10 nmthick), a protective layer 38 made of a dielectric material (ZnS—SiO₂,30 nm thick), and a reflection layer 39 made of a metal material (Agalloy, 100 nm thick). A transition of the phase-change layer 37 from theamorphous state to the crystalline state increases a reflectance. Itshould be noted that the transparent separation layer 4 is made of anultraviolet-hardening resin, and the protective layer 2 is made of apolycarbonate sheet with a thickness of 0.07 mm and anultraviolet-hardening resin with a thickness of 0.01.

[0054] In FIG. 1, the initialization device is composed of a spindlemotor 7, two optical heads 8 a and 8 b, a transport mount 9 on which theoptical heads are mounted, a transfer system 10 for moving the transportmount to a desired position, and a controller 11.

[0055]FIG. 2 illustrates a structure of the optical heads 8 a and 8 b.In the optical heads, a light beam emitted from a light source 12 madeof a semiconductor laser with a wavelength of 800 nm passes through acollimator lens 13, a beam splitter 14, a quarter-wavelength plate 15,and an objective lens 16, thereby being focused on a recording medium.The light beam is focused on the recording layer of the recording mediumby adjusting the position of the objective lens 16 by a voice coil 17.Light reflected from the recording layer passes again through theobjective lens 16 and the quarter-wavelength plate 15, and is reflectedby the beam splitter 14, thereby entering a detector 18, where it isconverted into electric signals for use in the control of the voice coil17.

[0056] In FIG. 1, light beams emitted from the optical heads 8 a and 8 bare focused on the recording layers 3 and 5, respectively, in theaforementioned manner. A spot of each light beam on the recording layeris in a prolonged ellipsoidal shape with a dimension of 100 μm in aradial direction of the recording medium and a dimension of 1 μm in acircumferential direction. Furthermore, the optical head 8 b ispositioned approximately 1 mm to a peripheral side of the recordingmedium relative to the optical head 8 a. For initialization, while thespindle motor 7 on which the recording medium 1 is placed is rotated,the optical heads 8 a and 8 b are moved by the transfer mount 9 from thecenter part to the periphery of the recording medium at a transfer pitchof 50 μm per one revolution of the recording medium, with the lightbeams maintained in a state of being focused on the recording layers.Here, the rotation of the spindle motor 7 is controlled so that a linearvelocity at the position of the optical head 8 a is kept substantiallyconstant. This process allows the recording layers 3 and 5 to beinitialized simultaneously, thereby allowing for the initialization of amultilayer recording medium with a plurality of recording layers withina short time.

[0057] Here, the optical head 8 b is positioned closer to the peripheryof the recording medium, i.e., with a deviation in the moving directionof the optical heads, relative to the optical head 8 a. Therefore, at acertain position on the recording medium, the initialization of therecording layer 3 is executed after the initialization of the recordinglayer 5. In other words, the initialization of the recording layer 5 iscarried out by the irradiation with the light beam having passed throughthe recording layer 3 in a non-initialized state (i.e., having a hightransmittance). Therefore, the following effect can be achieved: theinitialization of the recording layer 5 is implemented efficiently withthe attenuation of the light beam intensity caused by the recordinglayer 3 being suppressed to a small level, and furthermore, without theinfluence of initialization irregularities of the recording layer 3.Therefore, an optical information recording medium obtained in thepresent embodiment is a high-performance multilayer recording mediumhaving recording layers without initialization irregularities.

[0058] Here, the initialization irregularities are defined as partial orcomplete insufficient crystallization in the recording layers. Inrecording information, the jitter values indicative of qualities ofsignals vary upon each overwriting before the crystallization state ofthe recording layers are stabilized, thereby in some cases incurring aproblem in that the recorded information cannot be reproduced normally.The variation of the jitter values preferably is within 2%, and theinitialization irregularity indicates a state with a variation exceedingthe foregoing range.

[0059] An overwriting test was carried out with respect to a multilayerrecording medium according to the present embodiment, with a laser lighthaving a wavelength of 405 nm that was converged by an objective lenswith a NA of 0.85, at a linear velocity of 5.3 m/s. The signals wererandom signals recorded by the PWM recording method in which informationwas recorded according to the lengths of marks and spaces (that is, edgepositions of a front end and rear end of a mark) and modulated by the1-7 PP method with a reference clock T of 15.1 nsec. The laser light wasmodulated in a pulse form according to the signals to be recorded andprojected. The peak power and the bottom power of the same were set soas to be 10 mW and 4 mW, respectively, with respect to the recordinglayer 3, and to be 10 mW and 5 mW with respect to the recording layer 5.The peak power and the bottom power were selected so that the jittervalues when the random signals were overwritten ten times with differentpower values were minimized. Under the foregoing conditions, theoverwriting operation was carried out ten times, and a jitter value ofreproduction signals in each operation was measured. Consequently, thejitter values for the recording layers 3 and 5 both were in a range of10 to 11%. In other words, the variation of the jitter value was within1%, and no initialization irregularities were observed.

[0060] On the other hand, in the case where the medium was initializedby a conventional method in which the recording layer 3 closer to thelight beam irradiation side was initialized first and the identicaloverwriting test was carried out, the jitter value for the recordinglayer 5 varied in 10 to 14% in the first to tenth overwritingoperations, and thus, initialization irregularities were observed.

[0061] The following will describe a preferable positional relationshipbetween the optical heads 8 a and 8 b. FIG. 3 is a cross-sectional viewtaken in a radial direction of a portion of the recording medium 1irradiated with the light beam shown in FIG. 1. A position relationshipbetween the optical heads 8 a and 8 b preferably is set so that arelation shown below is satisfied:

z>(x/2)+(y/2)+(d·tan θ)

[0062] where

[0063] z represents a distance between radial locations on the recordingmedium 1 of the center lines of light beams 8 a′ and 8 b′ emitted fromthe optical heads 8 a and 8 b, respectively,

[0064] x represents a spot length in the radial direction of the lightbeam 8 a′ on the recording layer 3,

[0065] y represents a spot length in the radial direction of the lightbeam 8 b′ on the recording layer 5,

[0066] θ represents an incident angle of the light beam 8 b′, and

[0067] d represents a thickness of the transparent separation layer.

[0068] Here, the distance z between the radial locations preferably isin a range of 0.1 to 2 mm, the spot lengths x and y preferably are in arange of 50 to 200 μm each, the incident angle θ of the light beampreferably is in a range of 0.3 to 0.7, and the thickness d of thetransparent separation layer preferably is in a range of 10 to 60 μm.

[0069] In the above-described configuration, a length from a spot centerof the light beam 8 a′ on the recording layer 3 to an end of the samespot in the radial direction is x/2, a length from a spot center of thelight beam 8 b′ on the recording layer 3 to an end of the same spot inthe radial direction is (y/2)+(d·tan θ), and the distance z between thecenters of the spots of the light beams 8 a′ and 8 b′ is greater than asum of the foregoing two lengths, (x/2)+(y/2)+(d·tan θ). This means thatthe light beams 8 a′ and 8 b′ never overlap on the recording layer 3. Inother words, the light beam 8 b″ is projected on the recording layer 5without passing through the recording layer 3 in the initialized state.It should be noted that if the distance z between the radial locationsof the light beams 8 a′ and 8 b′ on the recording medium 1 is increasedexcessively, a wait time from the start of the initialization of therecording layer 5 to the start of the initialization of the recordinglayer 3 is prolonged, thereby causing the time required for theinitialization of the entirety to increase: the distance z can be set ina range tolerated according to a cycle time (a time for theinitialization of one recording medium). It should be noted the incidentangle θ of the light beam has a relationship expressed as:

NA=n·sin θ

[0070] where NA represents a numerical aperture of the objective lens,and n represents a refractive index of the substrate.

[0071] Furthermore, since the optical heads 8 a and 8 b are mounted onthe same transfer mount, the transfer mechanism and a control circuitfor the same may be identical to those in the case where one opticalhead is provided. Therefore, the following effects can be achieved: itis possible to suppress the growth in size of the device due to theprovision of two optical heads, and to set the two optical heads with aprecise positional relationship.

[0072] The following will describe an example of an optical head that isdesigned so that substrate thicknesses, through which the light beamsemitted from the optical heads 8 a and 8 b pass while being converged toform spots on the recording layers 3 and 5, respectively, are optimizedso as to make the formed spots have minimum diameters, respectively,according to the respective optimal distances therefrom to the recordinglayers 3 and 5.

[0073] In FIG. 1, an optical system for the optical head 8 a is designedso that the optimal substrate thickness for the same is 0.08 mm, whilean optical system for the optical head 8 b is designed so that theoptimal substrate thickness for the same is 0.12 mm. This means that alight beam emitted from the optical head 8 a to the recording medium 1focuses after passing the protective film with a thickness of 0.08 mm,that is, at the recording layer 3, without aberration. On the otherhand, a light beam emitted from the optical head 8 b to the recordingmedium 1 focuses after passing the protective film with a thickness of0.08 mm, the recording layer 3 with a thickness of 100 nm, and thetransparent separation layer with a thickness of 0.04 mm, that is, atthe recording layer 5, substantially without aberration. Therefore, itis possible for the optical heads to focus the light beams precisely onthe target recording layers, respectively, whereby the stableinitialization without irregularities can be achieved.

[0074] In the foregoing embodiment, the spots of the light beams emittedfrom the optical heads 8 a and 8 b and formed on the recording layershave the same shape, but the spots may have different shapes. Since therecording layers 3 and 5 have different thermal characteristics due totheir different structures, the layers have different temperaturedistributions when they are irradiated with the light beams, even if thespot shapes of the light beams irradiating the same are identical toeach other. Therefore, by forming the spots in shapes suitable for thethermal characteristics of the recording layers to be initialized, it ispossible to achieve the stable initialization without irregularities.

[0075] Furthermore, the foregoing embodiment is described referring to acombination of an initialization device having two optical heads and arecording medium having two recording layers, but three or more opticalheads may be provided. In the initialization of a recording medium withn recording layers, by matching the number of optical heads with thenumber of the recording layers, it is possible to achieve aninitialization speed n times higher than that in the case where only oneoptical head is provided. FIG. 4 illustrates a configuration in whichthree optical heads are provided, which is used with respect to aninformation recording medium 1′ having three recording layers. Theconfiguration is identical to that of the initialization device shown inFIG. 1, except that a third optical head 8 c is provided. Furthermore,the optical head 8 c has an identical configuration to that of theoptical heads 8 a and 8 b shown in FIG. 2. The optical head 8 c projectsa light beam to a recording layer positioned farthest from the beamirradiation side in the recording medium 1′ to initialize the recordinglayer, and it is arranged ahead relative to the moving direction amongthe three optical heads. This makes it possible to initialize arecording medium having three recording layers also within a short time,and to carry out the initialization of the recording layer positionedfarthest from the beam irradiation side without the influence ofinitialization irregularities of the recording layers positioned closerto the beam irradiation side, as in the case where a recording mediumhaving two recording layers is initialized by the initialization devicewith two optical heads shown in FIG. 1.

[0076] Furthermore, the moving direction of the optical heads during theinitialization may be directed from the periphery of the recordingmedium to the center of the same. In this case, the optical head 8 b isarranged on a side closer to the center of the recording medium relativeto the optical head 8 a, so as to be ahead relative to the optical headmoving direction.

Second Embodiment

[0077]FIG. 5 is a view illustrating a configuration of an initializationdevice of an optical information recording medium according to thesecond embodiment of the present invention, in a state of being usedwith respect to an information recording medium 1. The initializationdevice has an identical configuration as that of the initializationdevice for use with an optical information recording medium according tothe first embodiment, which is shown in FIG. 1, except for the opticalheads.

[0078] An optical head 8 d has an objective lens which is a single lens,a light source for emitting a crystallization beam for crystallizing arecording layer, and a light source for emitting a focus positioncontrol beam with an intensity such that the beam does not crystallizethe recording layer. FIG. 6 illustrates the configuration of the same.

[0079] The foregoing optical head includes light sources 40 and 41composed of semiconductor lasers with wavelengths of 800 nm and 680 nm,respectively.

[0080] In FIG. 6, a wavelength-selective mirror 46 transmits light witha wavelength at a level of the light emitted from the light source 40,and reflects light with a wavelength at a level of the light emittedfrom the light source 41. The light beam emitted from the light source40 passes through the wavelength-selective mirror 46, and is focused bythe objective lens 47 on a recording layer as the crystallization beam49. On the other hand, the light beam emitted from the light source 41passes through a collimator lens 43, a beam splitter 44, and aquarter-wavelength plate 45, is reflected by the wavelength-selectivemirror 46, and is focused by the objective lens 47 on a recording layerof the recording medium as the focus position control beam 50. The focusposition control beam 50 reflected from the recording layer again passesthrough the objective lens, is reflected by the wavelength-selectivemirror 46, passes through the quarter-wavelength plate 45, and isreflected by the beam splitter 44, thereby entering a detector 51, wherethe beam is converted into electric signals. The electric signals areused for controlling a voice coil 48 to adjust the position of theobjective lens 47 so that the crystallization beam 49 is focused on therecording layer.

[0081] The foregoing initialization device is effective in particularfor initializing a recording medium having a very low reflectance withrespect to the wavelength of the crystallization beam when the recordinglayer 3 is in an amorphous state. Normally, a multilayer recordingmedium is designed optically so that a recording layer closer to thebeam irradiation side has a greater transmittance, in order tofacilitate the recording and reproduction with respect to a recordinglayer farther from the beam irradiation side. Accordingly, the recordinglayer closer to the beam irradiation side has a smaller reflectance.

[0082] An optical information recording medium according to anembodiment of the present invention has a configuration identical to themultilayer recording medium shown in FIG. 12, except for the filmthicknesses of the layers.

[0083] The reflectances and transmittances of the recording layer havewavelength dependency, and with respect to light with a wavelength of800 nm, the recording layer 3 in the amorphous state has a reflectanceof 1% and a transmittance of 60%, while the recording layer 5 in thecrystalline state has a reflectance of 10%. Besides, with respect tolight with a wavelength of 680 nm, the recording layer 3 in theamorphous state has a reflectance of 3% and a transmittance of 50%,while the recording layer 5 in the crystalline state has a reflectanceof 8%.

[0084] The initialization of the recording layer 5 is carried out priorto the initialization of the recording layer 3, for instance, so as notto be influenced by initialization irregularities of the recording layer3. Therefore, when the recording layer 3 is initialized, the recordinglayer 3 is in the amorphous state while the recording layer 5 is in thecrystalline state.

[0085] Here, upon the irradiation with the crystallization beam with awavelength of 800 nm, 1% of the light is reflected from the recordinglayer 3, whereas 3.6% (10%×60%×60%) of the light is reflected from therecording layer 5. Therefore, in the case where a common technique suchas the knife-edge method or the astigmatism method is used for focusingthe light beam, an intensity ratio between a focus error signal obtainedfrom the recording layer 3 and a focus error signal obtained from therecording layer 5 is 1/3.6.

[0086]FIG. 7A illustrates the intensities of focus error signalsobtained from the recording layers: an arrow a indicates a sigmoidalcurve representing a signal from the recording layer 3; and an arrow bindicates a sigmoidal curve representing a signal from the recordinglayer 5. The focus error signal from the recording layer 5 has anintensity three times or more that of the focus error signal from therecording layer 3. Therefore, in the case of the recording layer 1 inwhich a distance between the recording layer 3 and the recording layer 5is as short as 0.04 mm, it is difficult to make the focus error signalsobtained from the recording layers 3 and 5, that is, the sigmoidalcurves, distinct from each other. As a result, the light beam is focusedon the recording layer 5 with a greater amount of reflected lighttherefrom, and it is difficult to focus the light beam on the recordinglayer 3 with a smaller amount of reflected light therefrom. Thus, it isdifficult to initialize the recording layer 3 by focusing thecrystallization beam thereon.

[0087] On the other hand, the initialization device in the presentembodiment employs the focus position control beam with a wavelength of680 nm to control the focus position of the crystallization beam. Withirradiation with a light beam with a wavelength of 680 nm, 3% of thelight is reflected from the recording layer 3, while 2.5% (10%×50%×50%)of the light is reflected from the recording layer 5.

[0088] Therefore, an intensity ratio between the focus error signalsobtained from the recording layers 3 and 5 is 3/2.5. FIG. 7B illustratesthe intensities of the focus error signals obtained from the recordinglayers: an arrow a indicates a sigmoidal curve representing a signalfrom the recording layer 3; and an arrow b indicates a sigmoidal curverepresenting a signal from the recording layer 5. The focus error signalfrom the recording layer 3 has a greater intensity, which ensures thefocusing on the recording layer 3.

[0089] Therefore, the optical information recording layer having aplurality of recording layers according to the present embodiment is ahigh-performance multilayer recording medium without initializationirregularities in recording layers.

[0090] It should be noted that the wavelengths of the crystallizationbeam and the focus position control beam can be set appropriatelyaccording to optical characteristics of a recording medium.

Third Embodiment

[0091]FIG. 8 is a view illustrating a configuration of an initializationdevice for use with an optical information recording medium according tothe third embodiment of the present invention, in a state of being usedwith respect to the information recording medium 1 having two recordinglayers, which is described in conjunction with the first embodiment. Theconfiguration is identical to that of the initialization device for anoptical information recording medium according to the first embodimentshown in FIG. 1 except that the optical heads 8 a and 8 b are mounted onindividual transfer mounts 9 and 9′, respectively.

[0092] Light beams emitted from the optical heads 8 a and 8 b arefocused on the recording layers 3 and 5, respectively. A spot of eachlight beam on the recording layer is in a prolonged ellipsoidal shapewith a dimension of 100 μm in a radial direction of the recording mediumand a dimension of 1 μm in a circumferential direction. Furthermore, theoptical head 8 b is positioned approximately 1 mm toward a peripheralside of the recording medium relative to the optical head 8 a. Forinitialization, while the spindle motor 7 on which the recording medium1 is placed is rotated, the optical heads 8 a and 8 b are moved by thetransfer mounts 9 and 9′ from the center part to the periphery of therecording medium at a transfer pitch of 50 μm per one revolution of therecording medium, with the light beams maintained in a state of beingfocused on the recording layers. Here, the rotation of the spindle motor7 is controlled so that a linear velocity at the position of the opticalhead 8 a is kept substantially constant. This process allows therecording layers 3 and 5 to be initialized simultaneously, therebyallowing for the initialization of a multilayer recording medium with aplurality of recording layers within a short time.

[0093] Here, the optical head 8 b is positioned closer to the peripheryof the recording medium, i.e., in the moving direction of the opticalheads, relative to the optical head 8 a. Therefore, at a given positionon the recording medium, the initialization of the recording layer 3 isexecuted after the initialization of the recording layer 5. In otherwords, the initialization of the recording layer 5 is carried out by theirradiation with the light beam having passed through the recordinglayer 3 in a non-initialized state (i.e., having a high transmittance).Therefore, the following effect can be achieved: the initialization ofthe recording layer 5 is implemented efficiently with the attenuation ofthe light beam intensity caused by the recording layer 3 beingsuppressed to a small level, and furthermore, without the influence ofinitialization irregularities of the recording layer 3.

[0094] Furthermore, by providing the optical heads 8 a and 8 b onindividual transfer mounts, an advantage can be achieved in that aconventional transfer mechanism can be employed, thereby reducing theproduction cost, though the device scale increases slightly.

[0095] It should be noted that the moving direction of the optical headsduring the initialization can be directed from the periphery to thecenter of the recording medium. In this case, the optical head 8 b isarranged on a side closer to the center of the recording medium relativeto the optical head 8 a, so as to be ahead relative to the optical headmoving direction.

[0096] Furthermore, as in the initialization device according to thesecond embodiment, two light sources that are for emitting acrystallization beam and for emitting a focus position control beam maybe provided in the optical head.

Fourth Embodiment

[0097] The fourth embodiment of the present invention relates to aninitializing method and an initialization device for initializing anoptical information recording medium having a plurality of recordinglayers, among which one recording layer closer to the beam irradiationside has a small reflectance in the amorphous state.

[0098]FIG. 9 is a view illustrating a configuration of an initializationdevice according to the present embodiment, in a state of being usedwith respect to the information recording medium 1 having two recordinglayers that is employed in the second embodiment. The initializationdevice has an identical configuration to that of the first embodimentshown in FIG. 1 except that a counter 52 for counting times ofup-and-down movements of the objective lens is provided. Theinitialization device includes the optical head shown in FIG. 2.

[0099] To initialize the recording layer 3, at an initial stage of theinitialization, the control by the controller 11 causes a light beamemitted from the optical head 8 a to have an intensity sufficient forthe initialization, and drives the voice coil 17 shown in FIG. 2 tocause the objective lens 16 to move up and down so that the focusposition of the light beam moves up and down a predetermined times. Arange in which the focus position of the light beam is moved is set soas to include at least the recording layer 3, and herein, it is set tobe 100 μm with respect to a predetermined position. Here, the counter 52counts the up-and-down movements of the objective lens, and the numberof the movements is controlled by the controller 11.

[0100] This action partially crystallizes the recording layer 3, and areflected light from the recording layer 3 that now has an increasedreflectance due to the partial crystallization is employed for causingthe optical head 8 a to focus on the recording layer 3. This allows thelight beam to be focused on the recording layer 3 surely forinitialization.

[0101] The number of the up-and-down movements of the objective lens isset according to a relative speed of the recording medium and theoptical head and the speed of the up-and-down movements, and itpreferably is set to be not less than two.

[0102] The foregoing initializing method is effective particularly forinitializing a recording medium in which the recording layer 3 in theamorphous state has a very low reflectance with respect to a light beamwith a wavelength at a level of the light beam emitted from the opticalhead 8 a.

[0103] An optical information recording medium according to anembodiment of the present invention has a configuration identical tothat of the multilayer recording medium shown in FIG. 12 except for thethickness of the layers, and the recording layer 3 has a reflectance of1% in the amorphous state and a reflectance of 6% in the crystallinestate, and a transmittance of 60% in the amorphous state and atransmittance of 30% in the crystalline state, whereas the recordinglayer 5 has a reflectance of 15% in the amorphous state and areflectance of 10% in the crystalline state. In a state in which therecording layer 5 has been initialized already while the recording layer3 is not initialized yet, 3.6% (10%×60%×60%) of light is reflected fromthe recording layer 5, while 1% of the light is reflected from therecording layer 3.

[0104] Therefore, in the case where a common technique such as theknife-edge method or the astigmatism method is used for focusing thelight beam, an intensity ratio between a focus error signal obtainedfrom the recording layer 3 and a focus error signal obtained from therecording layer 5 is 1/3.6, which is very small. FIG. 10A illustratesthe intensities of focus error signals obtained from the recordinglayers: arrows a and b indicate sigmoidal curves representing signalsfrom the recording layers 3 and 5, respectively.

[0105] The focus error signal from the recording layer 5 has anintensity three times or more that of the focus error signal from therecording layer 3. Therefore, in the case of the recording layer 1 inwhich a distance between the recording layer 3 and the recording layer 5is as short as 0.04 mm, it is difficult to make the focus error signalsobtained from the recording layers 3 and 5, that is, the sigmoidalcurves, distinct from each other. As a result, the light beam is focusedon the recording layer 5 with a greater amount of reflected lighttherefrom, and it is difficult to focus the light beam on the recordinglayer 3 with a smaller amount of reflected light therefrom. Thus, it isdifficult to initialize the recording layer 3 by focusing thecrystallization beam thereon.

[0106] However, by partially crystallizing the recording layer 3 with anintensity of the light beam sufficient for the initialization and withthe up-and-down movements of the focus position of the optical head 8 aat an initial stage of the initialization of the recording layer 3, 6%of the light is reflected from the recording layer 3 and 0.9%(10%×30%×30%) of the light is reflected from the recording layer 5.Accordingly, an intensity ratio between the focus error signals obtainedfrom the recording layers 3 and 5 is 6/0.9. FIG. 10B illustratesintensities of the focus error signals obtained from the recordinglayers: arrows a and b indicate sigmoidal curves representing signalsfrom the recording layers 3 and 5, respectively. The focus error signalfrom the recording layer 3 has a significantly greater intensity, whichensures the focusing on the recording layer 3.

[0107] Furthermore, FIG. 10C illustrates intensities of focus errorsignals obtained from the recording layers during the initialization:arrows a and b indicate sigmoidal curves representing signals from therecording layers 3 and 5, respectively. In the case of the presentembodiment in which a spot of each light beam on the recording layer hasa dimension of 100 μm in a radial direction of the recording medium anda transfer pitch is 50 μm, a half of the light beam spot falls on aninitialized portion. Therefore, 3.5% ((1%+6%)/2) of the light isreflected from the recording layer 3, while 2.25% ((3.6%+0.9%)/2) of thelight is reflected from the recording layer 5. Consequently, anintensity ratio between the focus error signals obtained from therecording layers 3 and 5 is 3.5/2.25, which is sufficiently great, andhence, it is possible to focus the light beam on the recording layer 3surely.

[0108] Therefore, the optical information recording medium having aplurality of recording layers according to the present embodiment is ahigh-performance multilayer recording medium without initializationirregularities in recording layers.

[0109] It should be noted that the relationship between the shape of thelight beam spot and the transfer pitch preferably is set so that thefocus error signal obtained from the recording layer 3 is greater thanthe focus error signal obtained from the recording layer 5.

[0110] Furthermore, as a technique for increasing a range subjected tothe initialization by decreasing a range of the up-and-down movements ofthe objective lens, a technique can be applied in which a light beamfirst is focused later on a recording layer that reflects much lightthereby producing a greater amplitude of a sigmoidal curve, so that aposition of the other layer is determined and thereafter initialized.

[0111] In the initialization of the recording layer 3, the followingoperation preferably is performed: a focusing operation is performed forfocusing the light beam emitted from the optical head 8 a on therecording layer 5, then, after stopping the focusing operation, thefocus position of the optical head 8 a is moved down 0.04 mm, which isequivalent to a distance between the recording layer 3 and the recordinglayer 5, and thereafter, the optical head 8 a is moved up and down whilecausing the same to emit a light beam with an intensity sufficient forthe initialization. This allows the focus position of the light beam topass surely through the recording layer 3 with short-distanceup-and-down movements of the optical head 8 a. This ensures the partialcrystallization of the recording layer 3 at an initial stage of theinitialization, and prevents the optical head 8 a from colliding againstthe recording medium. Here, in the case where the recording layer 3 iscrystallized partially by error before the light beam is focused on therecording layer 5, the light reflected from the recording layer 3increases while the light reflected from the recording layer 5decreases, which possibly causes the recording layer 3 to be mixed upwith the recording layer 5, thereby producing an error in determiningthe position of the optical head 8 a for initializing the recordinglayer 3. Therefore, in the case where the light beam is focused on therecording layer 5, the light beam preferably has an intensity such thatthe light beam does not crystallize the recording layer 3. This preventsthe partial crystallization of the recording layer 3 by error before thefocusing of the light beam on the recording layer 5. In other words, itis possible to set the position of the optical head 8 a in theinitialization of the recording layer 3.

Fifth Embodiment

[0112] An initialization device for use with an optical informationrecording medium according to the fifth embodiment of the presentinvention has a configuration identical to that of the initializationdevice for use with an optical information recording medium according tothe first embodiment shown in FIG. 1, except for the optical head. Theoptical head of the present embodiment is shown in FIG. 11. The opticalhead includes light sources 19 and 20 that are composed of semiconductorlasers emitting light beams with different wavelengths, which are 680 nmand 800 nm, respectively.

[0113] In FIG. 11, a wavelength-selective mirror 27 transmits light witha wavelength at a level of the light emitted from the light source 19,and reflects light with a wavelength at a level of the light emittedfrom the light source 20. The light beam emitted from the light source20 passes through a collimator lens 22, a beam splitter 24, and aquarter-wavelength plate 26, and is reflected by thewavelength-selective mirror 27, and is focused by an objective lens 28on the recording layer 5. The light beam reflected from the recordinglayer 5 again passes through the objective lens 28, is reflected by thewavelength-selective mirror 27, passes through the quarter-wavelengthplate 26, and is reflected by the beam splitter 24, thereby entering adetector 32, where the beam is converted into electric signals. Theelectric signals are used for controlling a voice coil 29. On the otherhand, the light beam emitted from the light source 19 passes through anoptical path correction system 30 composed of a liquid crystal element,a collimator lens 21, a beam splitter 23, and a quarter-wavelength plate25, transmits through the wavelength-selective mirror 27, and is focusedon the recording layer 3 by the objective lens 28. The light beamreflected from the recording layer 3 again passes through the objectivelens 28, the wavelength-selective mirror 27, and the quarter-wavelengthplate 25, and is reflected by the beam splitter 23, thereby entering adetector 31, where the beam is converted into electric signals. Theelectric signals are used for controlling the optical path correctionsystem 30.

[0114] The light beam emitted from the light source 20 is focused on therecording layer 5, which is one of the two recording layers of therecording medium 1, by the voice coil 29 that adjusts the position ofthe objective lens 28. When attempting to focus the light beam emittedform the light source 19 accurately on the other recording layer 3,deviations incurred by fluctuations of the position of the recordinglayer due to fluctuations of the recording medium 1 are common betweenthe recording layers 3 and 5, and therefore, such a deviation can becompensated by focusing the light beam from the light source 20 on therecording layer 5, as described above. However, deviations incurred bythickness irregularities or the like of the transparent separation layer4 are independent from the recording layer 5, and hence, such adeviation cannot be compensated by the foregoing method.

[0115] To compensate such a fluctuation peculiar to the recording layer3, the initialization device of the present embodiment varies theintensity and phase distribution of the light beam by operating theoptical path correction system 30, so as to adjust a focus positionaccording to the thickness irregularities of the transparent separationlayer 4. By so doing, the initialization device is capable of focusingthe light beam emitted from the light source 19 on the recording layer 3precisely.

[0116] Therefore, in the initialization device according to the presentembodiment, it is possible to focus a light beam emitted from one lightsource on a specific recording layer by moving an objective lens, and atthe same time, to focus a light beam emitted from another light sourceon another recording layer by means of the foregoing optical pathcorrection system. This makes it possible to focus light beams emittedfrom a plurality of light sources on respective target recording layersprecisely at the same time, even in the case where the transparentseparation layer 4 in the recording medium has a thickness differentfrom a designed one or has a thickness involving irregularities. Thus,the stable initialization without irregularities is achieved.Furthermore, in the case where the same objective lens is used, thefocus distance is shorter with respect to a light with a shortwavelength than with respect to a light with a long wavelength.Therefore, the light sources 19 and 20 may be configured so that thelight source 19 used for initializing the recording layer 3 of therecording medium closer to the light beam incidence surface emits alight with a shorter wavelength than that of the light emitted from thelight source 20 used for initializing the recording layer 5 farther fromthe light beam incidence surface, whereby the designing of the opticalsystem can be facilitated.

[0117] It should be noted that a liquid crystal element is employed asthe optical path correction system in the foregoing embodiment, but theoptical path correction system may be configured by employing a lenshaving a movable mechanism such as a piezoelectric element, and may bepositioned between the collimator lens 21 and the beam splitter 23.

[0118] Furthermore, the optical path correction system 30 may bepositioned on an optical path of the light beam emitted from the lightsource 20, and an operation for focusing the light beam emitted from thelight source 19 on the recording layer 5 is carried out by means of thevoice coil 29 that adjust a position of the objective lens 28, so thatthe focus position of the light beam emitted from the light source 20should be adjusted by operating the optical path correction system 30.

[0119] Furthermore, an example in which the position of the light beamprojected to the recording layer 5 is displaced in the optical headmoving direction relative to the position of the light beam projected tothe recording layer 3 so that the two light beams do not overlap eachother on the recording layers achieves an advantage in that therecording layer 5 can be initialized without the influence of theinitialization irregularities of the recording layer 3.

[0120] Furthermore, in the foregoing embodiment, the light beam emittedfrom the light source 19 used for initializing the recording layer 3that is closer to the light beam incidence surface of the recordingmedium is set so as to have a wavelength shorter than that of the lightbeam emitted from the light source 20 used for initializing therecording layer 5 that is farther from the light beam incidence surface,but the wavelength of the light source 19 may be set to be longer thanthe wavelength of the light source 20, for instance, in the case wherethe recording layer 3 is made of a material whose transmittanceincreases as the wavelength decreases.

[0121] Furthermore, the number of the light sources may be not less thanthree, and there is no need to match the same with the number of therecording layers of the recording medium.

[0122] Furthermore, semiconductor lasers emitting light beams withdifferent wavelengths are employed as light sources, but they may belight sources emitting light beams with the same wavelength in the casewhere another means is employed for separating reflected lights from therecording medium of light beams emitted from different light sources,for instance, by setting at different angles the optical paths of thelight beams emitted from the light sources 19 and 20 so that reflectedlights from the recording medium are focused at different positions inthe detector.

[0123] In the aforementioned first through fifth embodiments, thewavelengths of the light sources and the numerical aperture of theobjective lens can be designed appropriately according to the opticalcharacteristics of recording layers, a thickness of a substrate, etc.,of a recording medium as a target of the initialization.

[0124] Furthermore, the foregoing is described referring to aninformation recording medium in a disc form, but the present inventioncan be applied to multilayer recording media in other forms such as acard type.

[0125] The invention may be embodied in other forms without departingfrom the spirit or essential characteristics thereof. The embodimentsdisclosed in this application are to be considered in all respects asillustrative and not limiting. The scope of the invention is indicatedby the appended claims rather than by the foregoing description, and allchanges which come within the meaning and range of equivalency of theclaims are intended to be embraced therein.

What is claimed is:
 1. An optical information recording mediumcomprising a plurality of recording layers, a reflectance of at leastone of the plurality of the recording layers in a non-initialized statewith respect to a light beam for initialization being smaller than areflectance of the same in an initialized state with respect to thelight beam for initialization, wherein the recording layers areinitialized by irradiating, among the plurality of recording layers, therecording layer positioned farther from a light beam irradiation sidewith the light beam prior to irradiating the recording layer positionedcloser to the light beam irradiation side.
 2. The optical informationrecording medium according to claim 1, wherein the reflectance of atleast one of the plurality of recording layers in the non-initializedstate with respect to the light beam for initialization is not more than1/2 of the reflectance in the initialized state.
 3. The opticalinformation recording medium according to claim 1, wherein the recordinglayer positioned farther from the light beam irradiation side and therecording layer positioned closer to the light beam irradiation side areinitialized at the same time by one light-beam-projecting operation, byfocusing the light beam at different positions.
 4. An opticalinformation recording medium comprising a plurality of recording layers,a reflectance of at least one of the plurality of the recording layersin a non-initialized state with respect to a light beam forinitialization being smaller than a reflectance of the same in aninitialized state with respect to the light beam for initialization,wherein the initialized recording layers have no initializationirregularities.
 5. The optical information recording medium according toclaim 4, wherein the reflectance of the at least one of the plurality ofrecording layers in the non-initialized state with respect to the lightbeam for initialization is not more than 1/2 of the reflectance in theinitialized state.
 6. A method for manufacturing an optical informationrecording medium having a plurality of recording layers, comprising, forinitializing the optical information recording medium: projectingdifferent light beams to the plurality of recording layers,respectively, wherein at a position at a same distance in a radialdirection on the optical information recording medium, the recordinglayer positioned farther from a light beam irradiation side isirradiated with one light beam before the recording layer positionedcloser to the light beam irradiation side is irradiated with anotherlight beam so that the plurality of recording layers are initialized atthe same time by a light beam projecting operation in which the lightbeams are focused at different positions.
 7. The method formanufacturing an optical information recording medium according to claim6, wherein the light beams are emitted from a plurality of opticalheads, respectively, that are provided so as to face a same surface ofthe recording medium, and are moved in a predetermined initializationtraveling direction, and among the plurality of recording layers, therecording layer positioned farther from the light beam irradiation sideis irradiated with the light beam emitted from the optical headpositioned ahead relative to the initialization traveling direction, inthe initialization of the plurality of recording layers at the sametime.
 8. The method for manufacturing an optical information recordingmedium according to claim 7, wherein the light beams emitted from atleast two optical heads among the plurality of optical heads form spotsdifferent in shape from each other.
 9. The method for manufacturing anoptical information recording medium according to claim 6, wherein ininitializing at least one of the recording layers, a focus positioncontrol light beam different from a crystallization beam forcrystallizing a recording layer is employed for controlling a focusposition of the crystallization beam.
 10. The method for manufacturingan optical information recording medium according to claim 6, wherein ininitializing at least one of the recording layers, at an initial stageof the initialization, a focus position of an optical head is moved upand down, with a light beam having an intensity sufficient forinitialization, so that the recording layer is crystallized partially.11. The method for manufacturing an optical information recording mediumaccording to claim 6, wherein in initializing at least one of therecording layers, at an initial stage of the initialization, a focusposition is focused at a recording layer from which a greater reflectedlight amount is obtained than a reflected light amount obtained from thetarget recording layer, and thereafter a focus position of an opticalhead is moved a predetermined distance in a thickness direction of therecording medium so that the target recording layer is initialized. 12.The method for manufacturing an optical information recording mediumaccording to claim 6, wherein a focus position is focused at a recordinglayer from which a greater reflected light amount is obtained than areflected light amount obtained from the target recording layer, with alight beam having an intensity with which the target recording layer isnot crystallized, and thereafter a focus position of an optical head ismoved up and down with respect to a position at a predetermined distancetherefrom in a thickness direction of the recording medium so that thetarget recording layer is crystallized partially.
 13. The method formanufacturing an optical information recording medium according to claim6, wherein a focus position is focused at a recording layer from which agreater reflected light amount is obtained than from the targetrecording layer, with a light beam having an intensity with which thetarget recording layer is not crystallized, and thereafter a focusposition of an optical head is moved a predetermined distance in athickness direction of the recording medium, and the intensity of thelight beam is increased so as to be sufficient for initializing thetarget recording layer, so that the target recording layer isinitialized.
 14. The method for manufacturing an optical informationrecording medium according to claim 6, wherein a positional relationshipof the optical heads is set so as to satisfy a relationship expressedas: z>(x/2)+(y/2)+(d·tan θ) where x represents a spot width of a lightbeam for initializing the recording layer positioned closer to the lightbeam irradiation side, y and θ represent a spot width and an incidentangle, respectively, of a light beam for initializing the recordinglayer positioned farter from the light beam irradiation side, zrepresents a distance between the light beams, and d represents adistance between the recording layers.
 15. A method for manufacturing anoptical information recording medium having a plurality of recordinglayers, comprising initializing an optical information recording mediumwherein, in initializing at least one of the recording layers, a focusposition control light beam different from a crystallization beam forcrystallizing a recording layer is employed for controlling a focusposition of the crystallization beam.
 16. The method for manufacturingan optical information recording medium according to claim 15, whereinamong the plurality of recording layers, the recording layer positionedfarther from a light beam irradiation side is initialized prior toinitializing the recording layer positioned closer to the light beamirradiation side.
 17. The method for manufacturing an opticalinformation recording medium according to claim 15, wherein, while aplurality of optical heads provided to face a same surface of therecording medium are moved in a predetermined initialization travelingdirection, the recording layer positioned farther from the light beamirradiation side among the plurality of recording layers is irradiatedwith a light beam emitted from the optical head positioned aheadrelative to the initialization traveling direction, in theinitialization of the plurality of recording layers at the same time.18. An initialization device for initializing an optical informationrecording medium having a plurality of recording layers, comprising: aplurality of optical heads provided to face a same surface of arecording medium for irradiating different recording layers with lightbeams, respectively; and a transfer system for moving the optical headsin a predetermined initialization traveling direction, wherein among theplurality of optical heads, the optical head for irradiating a recordinglayer positioned farther from a light beam irradiation side among aplurality of recording layers is positioned ahead relative to theinitialization traveling direction.
 19. The initialization deviceaccording to claim 18, wherein a positional relationship of the opticalheads is set so as to satisfy a relationship expressed as:z>(x/2)+(y/2)+(d·tan θ) where x represents a spot width of a light beamfor initializing the recording layer positioned closer to the light beamirradiation side, y and θ represent a spot width and an incident angle,respectively, of a light beam for initializing the recording layerpositioned farther from the light beam irradiation side, z represents adistance between the light beams, and d represents a distance betweenthe recording layers.
 20. The initialization device according to claim18, wherein the light beams emitted from at least two optical headsamong the plurality of optical heads form spots different in shape fromeach other.
 21. The initialization device according to claim 18, furthercomprising a spindle motor for rotating the recording medium, whereinthe transfer system includes a transfer mount on which the optical headsare mounted and the transfer system moves the transfer mount so as tomove the optical heads to the predetermined position, wherein theplurality of optical heads are arranged so that light beams emitted fromthe optical heads are projected to the recording medium at differentpositions in a radial direction of the recording medium.
 22. Theinitialization device according to claim 21, wherein the plurality ofoptical heads are mounted on one transfer mount.
 23. The initializationdevice according to claim 21, wherein optimal substrate thicknesses forthe optical heads are different from each other, the optimal substratethicknesses being obtained by optimizing substrate thicknesses, throughwhich the light beams emitted from the optical heads pass while beingconverged to form spots on the recording layers, respectively, so as tomake the formed spots have minimum diameters, respectively.
 24. Theinitialization device according to claim 18, wherein at least one of theplurality of optical heads includes a driving system for moving anobjective lens equipped in the optical head, a counter for countingup-and-down movements of the objective lens, and a controller forcontrolling an intensity of the light beam emitted from the optical headand for controlling the driving system.
 25. An initialization devicecomprising at least one optical head and a transfer system for movingthe optical head in a predetermined initialization traveling direction,wherein at least one of the optical heads includes an objective lensthat is a single lens, a light source for emitting a crystallizationbeam for crystallizing a recording layer, and a light source foremitting a focus position control beam with an intensity with which therecording layer is not crystallized.
 26. The initialization deviceaccording to claim 25 comprising a plurality of optical heads providedfacing a same surface of the recording medium and projecting light beamsto different recording layers, respectively, wherein, among theplurality of optical heads, the optical head for irradiating therecording layer positioned farther from a light beam irradiation sideamong the plurality of recording layers is positioned ahead relative tothe initialization traveling direction.
 27. The initialization deviceaccording to claim 25, wherein the optical head includes an objectivelens and a plurality of light sources, the initialization device furthercomprising an optical path correction system positioned on an opticalpath of one light beam emitted from at least one of the optical sources.28. The initialization device according to claim 27, wherein the opticalpath correction system is a liquid crystal element or a lens.
 29. Theinitialization device according to claim 27, wherein the light sourcesemit light beams with different wavelengths from each other.